WO 2010063576 discloses a device for jetting droplets of a fluid at a high temperature, wherein the fluid is actuated by generating a Lorentz force in the fluid, further referred to as Lorentz actuation. WO2012168158 discloses a method for controlling temperature of such device.
To be able to generate a Lorentz force the fluid must comprise an electrically-conductive fluid. The device is suited to eject droplets of fluid at a high temperature, in particular of a molten metal or a molten semi-conductor, more in particular of metals having a high melting temperature (e.g. higher than about 1200 K), such as gold, silver, copper, titanium and the like. A Lorentz force is generated in the fluid, by applying an electric current pulse through the fluid, the fluid being positioned in a magnetic field. A direction and magnitude of the resulting force is related to the cross product of the electric current and the magnetic field vector: {right arrow over (F)}={right arrow over (I)}×{right arrow over (B)}.
To expel a droplet in a predetermined direction, it is preferred that the force generated in the fluid in the predetermined direction is optimized. Therefore, to obtain a maximal force in the fluid, a direct current pulse is applied to the fluid.
The direct current pulses used to eject a droplet also heat the fluid due to the Joule effect. The heating of the fluid may eventually also heat the jetting device. The generated heat (Q [W]) is proportional to the square of the applied current (I [A]) and the total resistance (R [Ω]) of the parts of the print head through which the actuation current runs, comprising the electrode resistance, the print head material resistance, the liquid metal resistance, and contact resistances (e.g. contacts between electrodes and print head material, contact of electrodes with the liquid metal).
The generated heat per unit of time (t [s]), during which a current is applied is therefore:
                              Q          t                =                                            I              2                        *            R                    =                                    V              2                        R                                              formula        ⁢                                  ⁢        1            
For the purpose of jetting droplets according to the above described method, the applied current may be very high (i.e. in the order of 100 A-200 A). If the electrically conductive fluid is ejected at a low frequency, (e.g. ˜1-10 Hz) and short pulse widths (e.g. <50 μs) the Joule effect may be small. However, to optimize productivity of printing systems for jetting droplets of an electrically conductive fluid, it is desired that the fluid is jetted at high frequencies (e.g. about 5 kHz or even higher). It is observed that at such high frequencies the average temperature of the jetting device, in particular of the nozzle can get very high. As long as the jetting device is made of suitable material capable of withstanding high temperatures, this does not have to be a problem.
However, the heat generated by the jetting device may not be constant over time. E.g. in between two subsequent direct current pulses, or in between print jobs, no direct current pulses may be applied to the electrically conductive fluid and hence, no Joule effect may occur to heat the fluid. Consequently, there may be substantial differences in the temperature of the jetting device and the electrically conductive fluid over time. Differences in temperature may cause fluctuations in jetting performances over time. This situation is undesired because it affects the jetting process, because the properties of molten metals and semi-conductors are temperature dependent.
Therefore a need exists for a method for adequately controlling the temperature of a jetting device for jetting droplets of an electrically conductive fluid at a high temperature. It is a further object of the invention to control the temperature of such jetting device without decreasing productivity.
It is therefore an object of the present invention to provide such a method.
It is another object of the present invention to provide a jetting device suitable for performing such a method.